In 2 Corinthians 3:3, the Corinthians are a letter from Christ written not with ink but with what?

Answer: The Spirit of the living God

They are a letter written not with ink but with the Spirit of the living God, not on tablets of stone but on human hearts.

“You yourselves are our letter, inscribed on our hearts, known and read by everyone. It is clear that you are a letter from Christ, the result of our ministry, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.”— 2 Corinthians 3:2-3 (BSB)